Synthesis and characterization of inorganic pigments based on transition metal oxynitrides

Tantalum zirconium oxynitrides in the Ta-Zr-O-N system of Ta(3-x)Zrx(O/N)(6) and Ta(3-x)ZrxN(5-x)Ox (0 <= x <= 0.6) compositions have been synthesized by the nitridation of the amorphous tantalum zirconium oxides, obtained by sol-gel technique, with flowing ammonia at temperatures in the range 1023-1123 K for several hours. Different coloration has been obtained, going from yellow to red, whose intensity is a function of the nitrogen content in each sample. The structure of these oxynitrides has been refined from powder x-ray diffraction by the Rietveld method. The present study shows that the baddeleyite phase is formed in the preliminary stages of the nitridation and then transformed to the Ta3N5-type phase. Below 1123 K, a baddeleyite-type structure with a monoclinic symmetry, space group P2(1)/c, is observed in these phases. When the nitration temperature increases, a marked increase in the nitrogen content takes place, and these materials adopt the Ta3N5 structure, space group C2/m. Diffuse reflectance spectra and CIE-LAB color coordinates suggest that the solid solutions Ta(3-x)Zrx(O/N)(6) and Ta(3-x)ZrxN(5-x)O-x (0 <= x <= 0.6) are expected to be promising candidates for new ecological pigments.